This invention relates in general to electrophotography and more specifically, to a novel method of preparing an electrophotographic imaging member.
In the art of electrophotography an electrophotographic imaging member containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The member is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer to form an electrostatic latent image. This latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer.
The photoconductive layer utilized in electrophotography may be a homogeneous single layer such as vitreous selenium or it may be a composite layer containing a photoconductor and other material. One type of composite photoconductive layer used in electrophotography is illustrated U.S. Pat. No. 3,121,006 which describes a number of layers comprising finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. In its present commercial form, the binder layer contains particles of zinc oxide, uniformly dispersed in a resin binder and coated on a paper backing.
In the particular examples described in U.S. Pat. No. 3,121,006, the binder comprises a material which is incapable of transporting injected charge carriers generated by the photoconductive particles for any significant distance. As a result, with the particular material disclosed, the photoconductive particles must be, in substantially continuous particle-to-particle contact throughout the layer in order to permit the charge dissipation required for cyclic operation. Therefore, with the uniform dispersion of photoconductive particles described, a relatively high volume concentration of the photoconductor, about 50 percent by volume, is usually necessary in order to obtain sufficient photoconductive particle-to-particle contact for rapid discharge. However, it has been found that high photoconductive loadings in the binder results in the physical continuity of the resin being destroyed, thereby significantly reducing the mechanical properties of the binder layer. Systems with high photoconductive loadings are often characterized as having little or no flexibility. On the other hand, when the photoconductor concentration is reduced appreciably below about 50 percent by volume, the photoinduced discharge rate is reduced, making high speed cyclic or repeated imaging difficult or impossible.
U.S. Pat. No. 3,037,861 to Hoegl et al teaches that poly(N-vinylcarbazole) exhibits some long-wavelength UV sensitivity and suggests that its spectral sensitivity can be extended into the visible spectrum by the addition of dye sensitizers. The Hoegl et al patent further suggests that other additives such as zinc oxide or titanium dioxide may be used in conjunction with poly(N-vinylcarbazole). In the Hoegl et al patent the poly(N-vinylcarbazole) is intended to be used as a photoconductor, with or without additive material which extend its spectral sensitivities.
In addition to the above, certain specialized layers particularly designed for reflex imaging have been proposed. For example, U.S. Pat. No. 3,165,405 to Hoesterey utilizes a two-layered zinc oxide binder structure for reflex imaging. The Hoesterey patent utilizes two separate contiguous photoconductive layers having different spectral sensitivities in order to carry out a particular reflex imaging sequence. The Hoesterey device utilizes the properties of multiple photoconductive layers in order to obtain the combined advantages of the separate photoresponse of the respective photoconductive layers.
It can be seen from a review of the conventional composite photoconductive layers cited above, that upon exposure to light, photoconductivity in the layered structure is accomplished by charge transport through the bulk of the photoconductive layer, as in the case of vitreous selenium (and other homogeneous layered modifications). In devices employing photoconductive binder structures which include inactive electrically insulating resins such as those described in U.S. Pat. No. 3,121,006, conductivity or charge transport is accomplished through high loadings of the photoconductive pigment and allow particle-to-particle contact on the photoconductive particles. In the case of photoconductive particles dispersed in a photoconductive matrix, as illustrated in U.S. Pat. No. 3,121,007, photoconductivity occurs through the generation and transport of charged carriers in both the photoconductive matrix and the photoconductive pigment particles.
Although the above patents rely upon distinct mechanisms of discharge through the photoconductive layer, they generally suffer from common deficiencies in that the photoconductive surface during operations is exposed to the surrounding environment, and particularly in the case of repetitive xerographic cyclic operation, where these photoconductive layers are susceptible to abrasion, chemical attack, heat and multiple exposure to light. These effects are characterized by a gradual deterioration of the electrical characteristics of the photoconductive layer resulting in the printing out of surface defects and scratches, which are localized areas of persistant conductivity which fail to retain an electrostatic charge.
In addition to the problems noted above, these photoreceptors require that the photoconductive comprise either 100 percent of the layer, as in the case of the vitreous selenium layer, or that they preferably contain a high proportion of photoconductive material in the binder configuration. The requirements of the photoconductive layer containing all or a major portion of a photoconductive material further restricts the physical characteristics of the final plate, drum or belt in that the physical characteristics such as flexibility and adhesion of the photoconductor to a supporting substrate are dictated by the physical properties of the photoconductor, and not by the resin or matrix material which is preferably present in a minor amount.
Another form of a composite photosensitive layer which has been considered by the prior art includes a layer of photoconductive material which is covered with a relatively thick plastic layer and coated on a supporting substrate.
U.S. Pat. No. 3,041,166 to Bardeen describes such a configuration in which a transparent plastic material overlies the layer of vitreous selenium on a supporting substrate. In operation, the free surface of the transparent plastic layer is electrostatically charged to a given polarity. This device is then exposed to activating radiation which generates a hole electron pair in the photoconductive layer. The electrons move through the plastic layer and neutralize positive charges on the free surface of the plastic layer thereby creating an electrostatic image. Bardeen, however, does not teach any specific plastic material which will function in this manner, and confines his examples to structures which use a photoconductor material for the top layer.
U.S. Pat. No. 3,598,582 describes a special purpose composite photosensitive device adapted for reflex exposure by polarized light. One embodiment which employs a layer of dichroic organic photoconductive particles arrayed and oriented on a supporting substrate and a layer of poly(N-vinylcarbazole) formed over the oriented layer of dichroic material. When charged and exposed to light polarized perpendicular to the orientation of the oriented layer, the oriented dichroic layer and poly(N-vinylcarbazole) layer are both sustantially transparent to the initial exposure light. When the polarized light strikes a white background of the document being copied, the light is de-polarized, reflected back through the device and absorbed by the dichroic photoconductive material. In another embodiment, the dichroic photoconductor is dispersed in oriented fashion through the layer of poly(N-vinylcarbazole).
Belgium Pat. No. 763,540, issued Aug. 26, 1971, discloses an electrophotographic member having at least two electrically operative layers. The first layer comprises a photoconductive layer which is capable of photogenerating charge carriers and injecting the photogenerated holes into a contiguous active layer. The active layer comprises a transparent organic material which is substantially non-absorbing in the spectral region of intended use, but which is "active" in that it allows injection of photogenerated holes from the photoconductive layer, and allows these holes to be transported through the active layer. The active polymers may be mixed with inactive polymers or nonpolymeric material.
Gilman, Defensive Publication Ser. No. 93,449, filed Nov. 27, 1970 published 8/8/8 OG 707 on July 20, 1970, Defensive Publication P888.013, discloses that the speed of an inorganic photoconductor such as amorphous selenium can be improved by including an organic photoconductor in the electrophotographic element. For example, an insulating resin binder may have TiO.sub.2 dispersed therein or may be a layer of amorphous selenium. This layer is overcoated with a layer of electrically inactive binder resin having an organic photoconductor such as 4,4'-bis(diethylamino)-2,2'-dimethyltriphenylmethane dispersed therein.
"Multi-Active Photoconductive Element", Martin A. Burwick, Charles J. Fox and William Light, Research Disclosure, Vol. 133; pages 38-43, May 1975, was published by Industrial Opportunities Limited, Homewell, Havant Hampshire, England. This disclosure relates to a photoconductive element having at least two layers comprising an organic photoconductor containing a charge transport layer in electrical contact with an aggregate charge generation layer. Both the charge generation layer and the charge transport layer are essentially organic compositions. The charge generation layer contains a continuous, electrically insulating polymer phase and a discontinuous phase comprising a finely divided, particulate, cocrystalline complex of (1) at least one polymer having an alkylidene diarylene group and a recurring unit and (2) at least one pyrylium-type dye salt. The charge transport layer is an organic layer which is capable of transporting injected charge carries of the charge generation layer. This layer may comprise an insulating resinuous material having 4,4'-bis(diethylamino)-2,2'-dimethyltriphenylmethane dispersed therein.
U.S. Pat. No. 3,265,496 discloses that N,N,N',N'-tetraphenylbenzidene may be used as photoconductive material in electrophotographic elements. This compound is not sufficiently soluble in the resin binders of the instant invention to permit a sufficient rate of photoinduced discharge.
Straughan, U.S. Pat. No. 3,312,548 in pertinent part, discloses a xerographic plate having a photoconductive insulating layer comprising a composition of selenium, arsenic and a halogen. The halogen may be present in amounts from about 10 to 10,000 parts per million. This patent further discloses a xerographic plate having a support, a layer of selenium and an overlayer of photoconductive material comprising a mixture of vitreous selenium, arsenic and halogen.
U.S. Pat. No. 3,265,496 is directed to an electrophotographic element comprising an electrically conductive support having coated thereon a photoconductive composition containing as the photoconductive substance, a polyfunctional tertiary amine selected from the group consisting of certain polytriphenylamines, poly-p-aminostyrenes, N,N,N',N'-tetraphenylbenzidenes and N,N,N',N'-tetraphenylenediamines. The photoconductive composition may be employed in a photoconductive layer with or without a binder. Numerous binders are described including polycarbonates. In addition, solvents of choice for coating compositions in U.S. Pat. No. 3,265,496 include benzene, toluene, acetone, 2-butanone, chlorinated hydrocarbons, e.g. methylene chloride, ethylene chloride, etc., ethers, e.g. tetrahydrofuran, or mixtures of these solvents, etc.
U.S. Pat. No. 3,615,415 pertains to a method of forming a heterogeneous photoconductive composition comprising the steps of dissolving in an organic solvent a pyrylium dye and polymeric material having an alkylidene diarylene moiety in the recurring unit, shearing the solution, forming a coating of the sheared solution and drying the coating to form a heterogeneous composition comprising a continuous phase of the polymeric material and a discontinuous phase of the combination of the dye and polymeric material. The polymeric material may be polycarbonates and polythiocarbonates, polyvinylethers, polyesters, polyalpha-olefins, phenolic resins and the like. Liquids useful as solvents for preparing coating solutions include a number of organic solvents such as aromatic hydrocarbons like benzene and toluene, ketones like acetone and ethylmethyl ketone, halogenated hydrocarbons like methylene chloride and ethylene chloride, furans like tetrahydrofurans, alkyl and aryl alcohols like methyl alcohol, ethyl alcohol and benzyl alcohol, as well as mixtures of such solvents.
U.S. Pat. No. 4,123,271 is directed to a process for the preparation of a photosensitive material which includes applying to a conductive substrate a coating composition prepared by adding a solution of alkli metal dichromate and a polar solvent of methanol or ethanol, to a composition of finely divided zine oxide and an electrically insulating organic synthetic resin binder in an aromatic solvent which is toluene and xylene. The polar solvent and aromatic solvent are miscible with each other with the resin binder being dissolved in the aromatic solvent and the dichromic compound being soluble in the polar solvent.
U.S. Pat. No. 3,946,129 relates to a process for preparing reprographic sheets for use in electrostatography. In this method, a coating composition is applied to a substrate, generally paper, out of a solution of a mixture of mutually miscible organic liquids, one being a solvent for the polymer and the other a non-solvent for the polymer and removing the organic liquids with most of the solvent being removed before a significant amount of non-solvent is removed.
U.S. Pat. No. 4,265,990 discloses a photosensitive member having photoconductive layer and a contiguous charge transport layer. The charge transport layer comprises a polycarbonate resin containing from about 25-75 percent by weight of one or more of a compound having the general formula: ##STR1## wherein X is selected from the group consisting of an alkyl group having from 1 to about 4 carbon atoms, e.g. methyl, ethyl, propyl, butyl, etc., and chlorine in the ortho, meta or para position. The diamine compound is applied to the photoconductive layer in a solution with polycarbonate resin and methylene chloride solvent.